In the recent drive for higher integration densities and operating speeds in LSI devices, the pattern rule is made drastically finer. The photolithography which is currently on widespread use in the art is approaching the essential limit of resolution determined by the wavelength of light.
As the exposure light used in the lithography for resist pattern formation, g-line (436 nm) or i-line (365 nm) from a mercury lamp was widely used in the past. Reducing the wavelength of exposure light was believed effective as the means for further reducing the feature size. For the mass production process of 64 MB dynamic random access memories (DRAM, processing feature size 0.25 μm or less) and later ones, the exposure light source of i-line (365 nm) was replaced by a KrF excimer laser having a shorter wavelength of 248 nm.
However, for the fabrication of DRAM with a degree of integration of 256 MB and 1 GB or more requiring a finer patterning technology (processing feature size 0.2 μm or less), a shorter wavelength light source was required. Photolithography using ArF excimer laser light (193 nm) has been under active investigation.
It was expected at the initial that the ArF lithography would be applied to the fabrication of 180-nm node devices. However, the KrF excimer lithography survived to the mass-scale fabrication of 130-nm node devices. So, the full application of ArF lithography started from the 90-nm node. The ArF lithography combined with a lens having an increased numerical aperture (NA) of 0.9 is considered to comply with 65-nm node devices.
For the next 45-nm node devices which required an advancement to reduce the wavelength of exposure light, the F2 lithography of 157 nm wavelength became a candidate. However, for the reasons that the projection lens uses a large amount of expensive CaF2 single crystal, the scanner thus becomes expensive, hard pellicles are introduced due to the extremely low durability of soft pellicles, the optical system must be accordingly altered, and the etch resistance of resist is low; the F2 lithography was postponed and instead, the early introduction of ArF immersion lithography was advocated (see Proc. SPIE Vol. 4690 xxix).
In the ArF immersion lithography, water is held between the projection lens and the wafer. Since water has a refractive index of 1.44 at 193 nm, pattern formation is possible even using a lens with NA of 1.0 or greater. Theoretically the NA of lens can be increased to 1.35. The resolution is improved by an increment of NA. A combination of a lens having NA of at least 1.2 with strong super-resolution technology suggests a way to the 45-nm node (see Proc. SPIE Vol. 5040, p 724).
However, as the circuit line width is reduced, the influence of contrast being degraded by acid diffusion becomes more serious for the resist material. The reason is that the pattern feature size is approaching the diffusion length of acid, and this causes a lowering of mask fidelity and a degradation of pattern rectangularity. Accordingly, to gain more benefits from a reduction of exposure light wavelength and an increase of lens NA, the resist material is required to increase a dissolution contrast or restrain acid diffusion, as compared with the prior art materials.
For restraining acid diffusion, an attempt was made to bind a photoacid generator in a base polymer. Among others, the polymer which is designed such that an acid generated upon light exposure is bound in its structure is advantageous in that acid diffusion is substantially restrained, and exposure dose dependency and pattern density dependency are improved (see JP-A 2008-133448).
However, the full restraint of acid diffusion leads to the inhibition of acid catalyzed reaction or chemical amplification, which may, in turn, lead to a lowering of sensitivity and dissolution contrast, with a risk of resolution being degraded. Particularly in the case of fine trench patterns and hole patterns which tend to have a low optical contrast, the lowering of dissolution contrast can degrade lithography properties such as depth of focus (DOF).
Meanwhile, sulfonium salts are generally used as the photoacid generator in the ArF lithography. Several attempts were made to improve dissolution contrast by modifying the cation structure of sulfonium salts. For example, JP 3632410 describes a sulfonium salt having 4-alkoxynaphthyl-1-tetrahydrothiophenium cation. JP 3995575 discloses a resist composition comprising a resin having a plurality of acid labile groups in combination with the sulfonium salt.
On the other hand, the attempt to improve dissolution contrast has some harmful influences including an influence on pattern profile. The enhancement of dissolution contrast is expected to form patterns of rectangular profile, but has a propensity that profile changes occur due to volatilization and re-deposition (or chemical flare) of components in a sub-surface layer of a resist film during the bake step. In particular, the trench pattern has a propensity that the resist film sub-surface layer is clogged by chemical flare, as compared with the line-and-space pattern. As a matter of concern, a shortage of DOF and edge roughness can be problems.
It would be desirable to have a resist composition which has properly suppressed acid diffusion, a high dissolution contrast, and resistance to chemical flare so that the resist composition may perform satisfactorily when processed by the lithography to form fine trench patterns and hole patterns.